Process for preparing hydroxy-substituted aromatic aldehydes

ABSTRACT

The present invention therefore relates to a process for preparing an aldehyde of the formula (I) 
     
       
         
         
             
             
         
       
     
     where one, two or all three radicals from the group of R 1 , R 3  and R 5  are hydroxyl, and that radical or those radicals from the group of R 1 , R 3  and R 5  which are not hydroxyl are each independently hydrogen, C 1 -C 8 -alkyl or C 6 -C 14 -aryl, and R 2  and R 4  are each independently hydrogen, C 1 -C 8 -alkyl, C 1 -C 8 -alkoxy or C 6 -C 14 -aryl, which comprises converting an aldehyde of the formula (II) 
     
       
         
         
             
             
         
       
     
     in which one, two or all three radicals from the group of R′ 1 , R′ 3  and R′ 5  are C 1 -C 8 -alkoxy, and that radical or those radicals from the group of R′ 1 , R′ 3  and R′ 5  which are not C 1 -C 8 -alkoxy are each independently hydrogen, C 1 -C 8 -alkyl or C 6 -C 14 -aryl, and R 2  and R 4  are each as defined for formula (I), at elevated temperature and elevated pressure in the presence of (C 1 -C 4 -alkyl) 2 -amine, and then isolating the reaction product of the formula (I).

The invention provides a process for preparing aromatic aldehydes whichpossess at least one hydroxyl group on the aromatic ring.

One example of a literature procedure for preparation ofhydroxy-substituted aromatic aldehydes is the O-demethylation ofaromatic methyl ethers. However, the industrial scale use of knownmethods for O-demethylation is limited since, for example, the reagentsused are expensive, promote corrosion and/or require very severereaction conditions, for example reaction by means of sodiumthioethoxide at 150° C. in dimethylformamide or reaction with sodium inliquid ammonia. Reactions with highly reactive reagents, for exampleboron tribromide, aluminum trichloride or boron trichloride, do lead toa rapid conversion but exhibit a significantly reduced selectivity assoon as two or three aromatic methoxy groups are present on the aromaticring. The use of strong Lewis acids or Lewis bases also becomesdifficult when the aromatic comprises further functional groups, forexample aldehyde, keto or benzylic alcohol groups, since side reactionscan occur more frequently in this arrangement.

The regioselective O-demethylation of poly-methoxy-substitutedbenzaldehydes is known from the literature, for example:

-   Demyttenaere et al., Tetrahedron 2002, 58, 2163-2166, uses AlCl₃ for    demethylation of 2,3,4-trimethoxybenzaldehyde. The demethylation is    effected in the presence of a large excess of AlCl₃ in the ortho and    para positions.-   Ren et al., Tetrahedron Asymm. 2002, 13, 1799-1804, uses a    piperidine-water mixture for demethylation of    2,3,4-trimethoxybenzaldehyde. Disadvantages are the long reaction    times and the slightly reduced applicability of this reaction to    only highly electron-rich compounds.-   Bhattacharya et al., Tetrahedron Lett. 2006, 565-567, uses NaSCN and    Triton-X 405 for demethylation of 3,4-dimethoxybenzaldehyde. This    conversion requires relatively high temperatures.-   Fang et al., J. Mol. Cat. A: Chemical 2007, 16-23, uses LiCl in    dimethylformamide under microwave irradiation for demethylation of    2,3,4-trimethoxybenzaldehyde and 3,4-dimethoxybenzaldehyde. The    conversion is performed in the presence of expensive LiCl. The    conversion also forms toxic methyl chloride.-   Prager and Tan, Tetrahedron Lett. 1967, 38, 3661-3664, use Lewis    acids, for example AlCl₃, for demethylation of    3,4-dimethoxybenzaldehyde. A disadvantage in this procedure is the    corrosion problems with this reagent, and the requirement for exact    setting of the molar ratio.-   Pearl et al., J. Am. Chem. Soc. 1952, 74, 4262-4263, uses sulfuric    acid for demethylation of 2,3,4-trimethoxybenzaldehyde.    Disadvantages are the corrosion problems with this reagent, and,    when this procedure is applied to 3,4-dimethoxybenzaldehyde, a    conversion in reverse regioselectivity compared to    2,3,4-trimethoxybenzaldehyde.

Proceeding from this prior art, it was an object of the presentinvention to provide a process for preparing a hydroxy-substitutedaromatic aldehyde, which can be performed in a manner which is easy tomanage in terms of process technology and with a high overall yield withmaximum regioselectivity on the industrial scale. It should be possibleto utilize inexpensive starting compounds and reagents which are easy torecover and have good reusability.

It has been found that, surprisingly, the reaction ofalkoxybenzaldehydes with dialkylamine at elevated temperature andelevated pressure leads to a rapid and also regioselectiveO-dealkylation.

The present invention therefore provides a process for preparing analdehyde of the formula (I)

where one, two or all three radicals from the group of R₁, R₃ and R₅ arehydroxyl, and that radical or those radicals from the group of R₁, R₃and R₅ which are not hydroxyl are each independently hydrogen,C₁-C₈-alkyl or C₆-C₁₄-aryl, and R₂ and R₄ are each independentlyhydrogen, C₁-C₈-alkyl, C₁-C₈-alkoxy or C₆-C₁₄-aryl, which comprisesconverting an aldehyde of the formula (II)

in which one, two or all three radicals from the group of R′₁, R′₃ andR′₅ are C₁-C₈-alkoxy, and that radical or those radicals from the groupof R′₁, R′₃ and R′₅ which are not C₁-C₈-alkoxy are each independentlyhydrogen, C₁-C₈-alkyl or C₆-C₁₄-aryl, and R₂ and R₄ are each as definedfor formula (I), at elevated temperature and elevated pressure in thepresence of (C₁-C₄-alkyl)₂-amine, and then isolating the reactionproduct of the formula (I).

The process according to the invention is notable for a surprisinglyhigh regioselectivity. For example, by the process according to theinvention, a 3,4,5-trimethoxybenzaldehyde is converted very selectivelyto the 4-hydroxy-3,5-dimethoxybenzaldehyde.

Examples of useful radicals for the R₁, R₂, R₃, R₄, R₅, R′₁, R′₃ and R′₅radicals as C₁-C₈-alkyl include: methyl, ethyl, n-propyl, isopropyl,n-butyl, sec-butyl, tert-butyl, isobutyl, n-pentyl, n-hexyl or n-octyl,where the C₁-C₈-alkyl radicals mentioned may be substituted, forexample, by halogen, such as chlorine, fluorine or bromine, or hydroxyl.Preference is given to C₁-C₄-alkyl radicals and particular preference toC₁-C₃-alkyl radicals. Very particularly preferred alkyl radicals aremethyl and ethyl.

Preferred useful radicals for the R₁, R₂, R₃, R₄, R₅, R′₁, R′₃ and R′₅radicals as C₆-C₁₄-aryl are the phenyl or naphthyl radical, whichradicals may be further substituted on the aromatic system, for exampleby halogen, for example fluorine, chlorine or bromine, C₁-C₄-alkyl, forexample methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl,tert-butyl and isobutyl, C₁-C₄-alkoxy, for example methoxy, ethoxy,n-propoxy, isopropoxy, n-butoxy and tert-butoxy.

Examples of useful radicals for the R₂, R₄, R′₁, R′₃ and R′₅ radicals asC₁-C₈-alkoxy include: methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy,tert-butoxy, n-pentyloxy and n-octyloxy. Preference is given toC₁-C₄-alkoxy radicals, and particularly preferred radicals are methoxyand ethoxy. For the R′₁, R′₃ and R′₅ radicals, the methoxy radical isvery particularly preferred. For the R₂ and R₄ radicals, veryparticularly preferred C₁-C₄-alkoxy radicals are methoxy and ethoxy.

The inventive conversion is effected preferably at a temperature in therange between 40 and 300° C., more preferably in the range between 60and 250° C., and most preferably in the range between 80 and 160° C.

The process according to the invention is performed under a pressurebetween 1 and 100 bar, preference being given to a pressure between 2and 60 bar. Very particular preference is given to a pressure between 2and 20 bar.

The reaction can be performed over a prolonged period, the reaction timeof the conversion being in the range between 1 and 48 hours. Preferenceis given to a reaction time between 2 and 18 hours, and the reactiontime is most preferably between 6 and 15 hours.

The process according to the invention is performed in the presence of(C₁-C₄-alkyl)₂ amine. Especially useful dialkylamines include:dimethylamine, diethylamine, di-n-propylamine, diisopropylamine. Theamines are more preferably dimethylamine and diethylamine. Veryparticular preference is given to dimethylamine.

In a likewise preferred configuration of the process according to theinvention, the conversion is effected in the presence of thedialkylamine in aqueous solution. Dimethylamine has very good watersolubility. It is thus possible to use commercial aqueous solutions ofdimethylamine in water. Commercial solutions of dimethylamine in watercomprise approx. 35 to 65 mol % dimethylamine. Diethylamine is liquidand can likewise be used as an aqueous solution. Di-n-propylamine anddiisopropylamine can likewise be used as an aqueous solution. In apreferred embodiment of the process according to the invention,dimethylamine is used in aqueous solution. The dimethylamine content ofthe aqueous solution is preferably 40 to 60 mol %.

In a further embodiment, the process according to the invention can beperformed in the presence of one or more further solvents. Usefulfurther solvents include, in particular, polar solvents, for exampleacetonitrile, acetone, ethyl acetate, or else alcohols, for examplemethanol, ethanol.

In a further preferred embodiment of the process according to theinvention, an aldehyde of the formula (II) in which one, two or allthree radicals from the group of R′₁, R′₃ and R′₅ are C₁-C₄-alkoxy,especially methoxy, and that radical or those radicals from the group ofR′₁, R′₃ and R′₅ which are not C₁-C₄-alkoxy, especially methoxy, areeach independently hydrogen, C₁-C₄-alkyl or C₆-C₁₀-aryl, and R₂ and R₄are each independently hydrogen, C₁-C₈-alkyl, C₁-C₈-alkoxy orC₆-C₁₄-aryl, is used.

In a particularly preferred embodiment of the process according to theinvention, an aldehyde of the formula (II) in which one, two or allthree radicals from the group of R′₁, R′₃ and R′₅ are methoxy, and thatradical or those radicals from the group of R′₁, R′₃ and R′₅ which arenot methoxy are each independently hydrogen, C₁-C₄-alkyl or C₆-C₁₀-aryl,and R₂ and R₄ are each hydrogen, C₁-C₄-alkyl, C₁-C₄-alkoxy orC₆-C₁₀-aryl, is used.

In a very particularly preferred embodiment of the process according tothe invention, an aldehyde of the formula (IIa)

in which R′₁ and R′₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl, and X is C₁-C₄-alkyl, especially methyl, isconverted to an aldehyde of the formula (Ia)

in which R₁ and R₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl.

In a likewise very particularly preferred embodiment of the processaccording to the invention, an aldehyde of the formula (IIb)

in which R′₃ and R′₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl, and X is C₁-C₄-alkyl, especially methyl, isconverted to an aldehyde of the formula (Ib)

in which R₃ and R₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl.

In an even more preferred embodiment of the process according to theinvention, an aldehyde of the formula (IIa) in which R′₁, R′₅, R₂ and R₄are each hydrogen, and X is C₁-C₄-alkyl, especially methyl, is convertedto an aldehyde of the formula (Ia) in which R₁, R₂, R₄ and R₅ are eachhydrogen.

In a further even more preferred embodiment of the process according tothe invention, an aldehyde of the formula (IIa) in which R′₁, R′₅ and R₄are each hydrogen and R₂ is C₁-C₄-alkoxy, especially methoxy, and X isC₁-C₄-alkyl, especially methyl, is converted to an aldehyde of theformula (Ia) in which R¹, R⁴ and R⁵ are each hydrogen and R₂ isC₁-C₄-alkoxy, especially methoxy.

In a likewise even more preferred embodiment of the process according tothe invention, an aldehyde of the formula (IIa) in which R′₁ and R′₅ areeach hydrogen and R₂ and R₄ are each C₁-C₄-alkoxy, especially methoxy,and X is methyl, is converted to an aldehyde of the formula (Ia) inwhich R₁ and R₅ are each hydrogen and R₂ and R₄ are each C₁-C₄-alkoxy,especially methoxy.

In a further even more preferred embodiment of the process according tothe invention, an aldehyde of the formula (IIc).

in which R₂ and R₄ are each independently hydrogen or C₁-C₄-alkoxy,preferably hydrogen, methoxy or ethoxy, and X is C₁-C₄-alkyl, preferablymethyl, is converted to an aldehyde of the formula (Ic)

in which R₂ and R₄ are each as defined for formula (IIc). The conversionis effected preferably in the presence of dimethylamine at a temperaturein the range between 130 and 150° C. and a pressure of 5 to 15 bar.After a reaction time of 12 to 15 hours, the conversion has ended.

Useful starting compounds for the process according to the inventioninclude, for example, the following compounds of the formula (II):2-methoxybenzaldehyde, 4-methoxybenzaldehyde, 2-ethoxybenzaldehyde,4-ethoxybenzaldehyde, 3,4-dimethoxybenzaldehyde,3,4-diethoxybenzaldehyde, 3,4,5-trimethoxybenzaldehyde,3-methyl-4-methoxybenzaldehyde, 3-ethyl-4-methoxybenzaldehyde,2-methyl-5-methoxybenzaldehyde, 2,5-dimethyl-4-methoxybenzaldehyde,2-phenyl-4-ethoxybenzaldehyde.

The inventive conversion is effected under pressure, and sopressure-resistant apparatus has to be used. Useful pressure-resistantapparatus includes commercial autoclaves or else continuouspressure-resistant apparatus, for example tubular reactors.

Autoclaves must be able to withstand relatively high pressures. Typicallaboratory autoclaves withstand approx. 150 bar. The outer walls arethick and often consist of stainless steels in order to preventcorrosion and not to contaminate the charge. For the performance ofreactions with aggressive chemicals, autoclaves with internal Tefloncoatings are obtainable. Specific designs allow pressures up to 7000 barand temperatures above 600° C. In the laboratory, autoclaves with avolume of a few milliliters up to several liters are widespread, whichtypically possess a manometer, a thermometer and a gas valve.

The reaction products obtained by the process according to the inventionare recovered by working up the reaction mixture by customary processes,for example extraction, distillation and/or crystallization. Forexample, excess dimethylamine and water are removed under reducedpressure. Subsequently, a water-immiscible solvent is added, for examplediethyl ether, methyl tert-butyl ether or the like, the mixture isacidified with an acid, for example 30% HCl in water, and, after thephase separation and after removal of all volatile constituents of theorganic phase, the product can be obtained in good yield, optionallyafter distillation or crystallization.

The conversion to be performed in accordance with the invention can beperformed either batchwise or continuously. For example, in thebatchwise case, the conversion can be undertaken in such a way that thealdehyde of the formula (II) and, for example, an aqueous solution ofthe dialkylamine, optionally in the presence of a further solvent, forexample methanol, are initially charged in a suitable reaction vessel,for example an autoclave, and the conversion is undertaken at elevatedtemperature and under elevated pressure. On completion of the reaction,as described above, excess dialkylamine and water are removed underreduced pressure, a water-insoluble solvent, for example methyltert-butyl ether, is added, the mixture is acidified with, for example,30% HCl in water, and, after phase separation and removal of allvolatile constituents of the organic phase, the reaction product of theformula (I) is isolated from the reaction mixture obtained, by suitableseparating processes. The sequence of contacting of the individualreaction components is not critical and can be varied according to theparticular process technology configuration.

In a preferred embodiment, the dealkylation to be performed inaccordance with the invention, especially the demethylation, of thealdehyde of the formula (II) in the presence of the dialkylamine isperformed continuously, for example in a continuous tubular reactor, atelevated temperature and elevated pressure. For this purpose, it ispossible to prepare, for example, a mixture of the starting materials tobe converted, the aldehyde of the formula (II) and the dialkylamine,optionally as an aqueous solution, and to bring this mixturecontinuously into mutual contact. For this purpose, the startingcomponents selected, aldehyde of the formula (II) and dialkylamine, canbe introduced into a tubular reactor and the starting materials can beintroduced continuously into it, and the reaction mixture can bedischarged continuously.

The invention is illustrated in detail by the examples which followwithout restricting it thereto. In the examples, all figures in % are %by weight.

EXAMPLE 1

A 100 ml glass autoclave is charged with 8.84 g (65.0 mmol) of4-methoxybenzaldehyde and 51.2 g (455 mmol) of dimethylamine (40%solution in water). The autoclave is closed and the biphasic reactionmixture is stirred at 140° C. (autogenous pressure 7.5 bar) for 15 h.Excess dimethylamine and water are removed under reduced pressure,methyl tert-butyl ether is added and the mixture is acidified with 30%hydrochloric acid in water. After phase separation and removal of allvolatile constituents of the organic phase, 6.5 g (0.53 mmol, 81% yield)of 4-hydroxybenzaldehyde are obtained.

EXAMPLE 2

A metal autoclave is charged with 14.0 g (84.0 mmol) of3,4-dimethoxybenzaldehyde and 120 g (1.0 mol) of dimethylamine (40%solution in water). The autoclave is closed and the biphasic reactionmixture is stirred at 140° C. (autogenous pressure 14 bar) for 10 h.Excess dimethylamine and water are removed under reduced pressure,methyl tert-butyl ether is added and the mixture is acidified with 30%hydrochloric acid in water. After phase separation and removal of allvolatile constituents of the organic phase, 8.5 g (56.0 mmol, 67% yield)of 4-hydroxy-3-methoxybenzaldehyde are obtained.

EXAMPLE 3

A 100 ml glass autoclave is charged with 11.8 g (60.0 mmol) of3,4,5-trimethoxybenzaldehyde and 47.2 g (420 mmol) of dimethylamine (40%solution in water). The autoclave is closed and the biphasic reactionmixture is stirred at 140° C. (autogenous pressure 10 bar) for 18 h.Excess dimethylamine and water are removed under reduced pressure,methyl tert-butyl ether is added and the mixture is acidified with 30%hydrochloric acid in water. After phase separation and removal of allvolatile constituents of the organic phase,4-hydroxy-3,5-dimethoxybenzaldehyde is obtained in a yield of 72%.

1.-19. (canceled)
 20. A process for preparing an aldehyde of the formula(I)

wherein one, two or all three radicals from the group of R₁, R₃ and R₅are hydroxyl, and that radical or those radicals from the group of R₁,R₃ and R₅ which are not hydroxyl are each independently hydrogen,C₁-C₈-alkyl or C₆-C₁₄-aryl, and R₂ and R₄ are each independentlyhydrogen, C₁-C₈-alkyl, C₁-C₈-alkoxy or C₆-C₁₄-aryl, which comprisesconverting an aldehyde of the formula (II)

in which one, two or all three radicals from the group of R′₁, R′₃ andR′₅ are C₁-C₈-alkoxy, and that radical or those radicals from the groupof R′₁, R′₃ and R′₅ which are not C₁-C₈-alkoxy are each independentlyhydrogen, C₁-C₈-alkyl or C₆-C₁₄-aryl, and R₂ and R₄ are each as definedfor formula (I), at elevated temperature and elevated pressure in thepresence of (C₁-C₄-alkyl)₂-amine, and then isolating the reactionproduct of the formula (I).
 21. The process according to claim 20,wherein the conversion is effected at a temperature between 40 and 300°C.
 22. The process according to claim 20, wherein the conversion iseffected at a temperature between 80 and 160° C.
 23. The processaccording to claim 20, wherein the conversion is effected at a pressurebetween 1 and 100 bar.
 24. The process according to claim 20, whereinthe conversion is effected at a pressure between 2 and 20 bar.
 25. Theprocess according to claim 20, wherein the reaction time is between 1and 48 hours.
 26. The process according to claim 20, wherein thereaction time is between 6 and 15 hours.
 27. The process according toclaim 20, wherein the conversion is effected in the presence ofdimethylamine or diethylamine.
 28. The process according to claim 20,wherein the conversion is effected in the presence of dimethylamine. 29.The process according to claim 20, wherein the conversion is effected inthe presence of the (C₁-C₄-alkyl)₂-amine in aqueous solution.
 30. Theprocess according to claim 29, wherein the conversion is effected in thepresence of dimethylamine in aqueous solution.
 31. The process accordingto claim 20, wherein the aldehyde of the formula (II) in which one, twoor all three radicals from the group of R′₁, R′₃ and R′₅ areC₁-C₄-alkoxy, and that radical or those radicals from the group of R′₁,R′₃ and R′₅ which are not C₁-C₄-alkoxy are each independently hydrogen,C₁-C₄-alkyl or C₆-C₁₀-aryl, and R₂ and R₄ are each as defined forformula (I) is used.
 32. The process according to claim 20, wherein thealdehyde of the formula (II) in which one, two or all three radicalsfrom the group of R′₁, R′₃ and R′₅ are C₁-C₄-alkoxy, and that radical orthose radicals from the group of R′₁, R′₃ and R′₅ which are notC₁-C₄-alkoxy, are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each hydrogen, C₁-C₄-alkyl, C₁-C₄-alkoxyor C₆-C₁₀-aryl, is used.
 33. The process according to claim 32, whereinan aldehyde of the formula (IIa)

in which R′₁ and R′₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl, and X is C₁-C₄-alkyl, is converted to analdehyde of the formula (Ia)

in which R₁ and R₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl.
 34. The process according to claim 32,wherein an aldehyde of the formula (I),

in which R′₃ and R′₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl, and X is C₁-C₄-alkyl, is converted to analdehyde of the formula (Ib)

in which R₃ and R₅ are each independently hydrogen, C₁-C₄-alkyl orC₆-C₁₀-aryl, and R₂ and R₄ are each independently hydrogen, C₁-C₄-alkyl,C₁-C₄-alkoxy or C₆-C₁₀-aryl.
 35. The process according to claim 33,wherein the aldehyde of the formula (IIa) in which R′₁, R′₅, R₂ and R₄are each hydrogen, and X is C₁-C₄-alkyl, is converted to an aldehyde ofthe formula (Ia) in which R₁, R₂, R₄ and R₅ are each hydrogen.
 36. Theprocess according to claim 33, wherein an aldehyde of the formula (IIa)in which R′₁, R′₅ and R₄ are each hydrogen and R₂ is C₁-C₄-alkoxy and Xis C₁-C₄-alkyl, is converted to an aldehyde of the formula (Ia) in whichR₁, R₄ and R₅ are each hydrogen and R₂ is C₁-C₄-alkoxy.
 37. The processaccording to claim 33, wherein the aldehyde of the formula (IIa) inwhich R′₁ and R′₅ are each hydrogen and R₂ and R₄ are each C₁-C₄-alkoxy,and X is methyl, is converted to an aldehyde of the formula (Ia) inwhich R₁ and R₅ are each hydrogen and R₂ and R₄ are each C₁-C₄-alkoxy.38. The process according to claim 33, wherein the aldehyde of theformula (IIa) in which R′₁ and R′₅ are each hydrogen and R₂ and R₄ areeach methoxy, and X is methyl, is converted to an aldehyde of theformula (Ia) in which R₁ and R₅ are each hydrogen and R₂ and R₄ are eachmethoxy.
 39. The process according to claim 33, wherein an aldehyde ofthe formula (IIc)

in which R₂ and R₄ are each independently hydrogen or C₁-C₄-alkoxy, andX is C₁-C₄-alkyl is converted to an aldehyde of the formula (Ic)

in which R₂ and R₄ are each as defined for formula (IIc).
 40. Theprocess according to claim 39, wherein R₂ and R₄ are each independentlyhydrogen, methoxy or ethoxy, and X is methyl.
 41. The process accordingto claim 20, wherein the conversion is effected continuously.